The present invention relates to a process for recovering the plutonium contained in solid waste, such as process, technological and laboratory waste.
It is known that one of the problems frequently encountered in nuclear installations is that of recovering the plutonium and/or neptunium present in the solid waste resulting either from the production of nuclear fuel elements or from the processing of the irradiated nuclear fuel. Such waste can be constituted by ash resulting from the incineration of highly plutonium contaminated fuel waste to 800.degree. to 900.degree. C. so that calcined ash is obtained in which the plutonium is in the form of oxide or other refractory compounds, i.e. a form which is difficult to recover in solution. Solid waste is also obtained, which contains plutonium in oxide form during the decanning of irradiated nuclear fuel elements, because a by no means negligible plutonium oxide quantity remains fixed to the fuel can sections. Finally, certain laboratory waste, particularly that based on plastic materials can also be contaminated by plutonium oxide, which it is difficult to recover by dissolving.
Hitherto, the most frequently used method for dissolving the plutonium oxide contained in the waste, such as the incineration ash of fuel waste, was based on the catalytic effect of fluoride ions making it possible to dissolve the plutonium in nitrohydrofluoric solution. This, in such waste, the main and most disturbing impurity is silica, which should be eliminated as from the start of processing. To this end, the ash is treated cold for 24 hours in 10N hydrofluoric acid in order to bring about a maximum solubilization of the silica. This is followed by subjecting the residue to the action of a mixture of 6M of nitric acid and 2N hydrofluoric acid in boiling form. This is followed by the expulsion of the final traces of silica in the form SiF.sub.6 H.sub.2, which distils and finally the plutonium fluoride is solubilized by a mixture of 4M nitric acid and 1M aluminium nitrate at boiling.
Thus, this method requires the use of very corrosive reagents and leads to the production of highly fluorinated effluents, whose treatment causes problems.
Consideration has also been given to the recovery of the plutonium contained in waste by aluminothermy, i.e. by reducing the plutonium oxide with aluminium, which leads to an alloy PuA1, which can then be dissolved in a HNO.sub.3 --HF medium. However, this method does not give satisfactory results, because the recovery level of the plutonium or uranium contained in the ash does not exceed 50%. It is also possible to recover the plutonium by alkaline fusion in a bath of Na.sub.2 O.sub.2 and NaOH. After fusion, the vitreous mass containing the plutonium is immersed in water which is acidified with a nitric acid excess in order to recover the plutonium in the solution. However, the solution obtained is not stable, and at the end of a certain time a flaky white SiO.sub.2 precipitate forms in the solution and partly entrains the plutonium.
Consideration can also be given to the conversion of the plutonium oxide contained in the ash into plutonium hexafluoride by reacting with gaseous fluorine, but in this case, in view of the considerable reactivity of the ash, the latter tends to solidify and it is not possible to obtain a satisfactory plutonium recovery level.
Thus, none of the presently known processes makes it possible to recover the plutonium from waste under satisfactory conditions.